Simultaneous diffusion, adsorption and reaction in FCC catalysts

C. BIDABEHERE; U. SEDRAN

INDUSTRIAL & ENGINEERING CHEMICAL RESEARCH

AMER CHEMICAL SOC

Lugar: Washington; Año: 2001 vol. 40 p. 530 - 535

0888-5885

Simultaneous diffusion, adsorption, and reaction that take place inside the zeolitic componentof equilibrium commercial fluid catalytic cracking (FCC) catalysts were described by means ofheterogeneous models. n-Hexadecane was used as a test reactant at high temperatures (440-550 °C) over two different equilibrium catalysts under very short contact times up to 10 s in aRiser Simulator reactor. The system?s parameters were obtained by fitting the model to thereactant?s gas-phase concentration versus reaction time data. When zeolite intracrystallinediffusion was first assumed as the controlling mechanism for mass transfer, its energy ofactivation resulted close to the heat of adsorption, suggesting that diffusion in the zeolitemicropores could be indeed controlling. The solution under this new approach led to the obtentionof parameters that were consistent with the existence of strong diffusion limitations for thereaction and with lower activity in the low unit cell size catalyst. Diffusion, which would be anonactivated process, had coefficients that were essentially the same in both catalysts, whilethe energies of activation of the reaction were different and reflected the higher relativeimportance of the mechanism of monomolecular cracking in the more dealuminated catalyst.The need for a careful assessment of adsorption parameters in FCC catalysts was confirmed bythe fact that their magnitudes change significantly as a function of temperature, with adsorptionbeing somewhat stronger on the higher unit cell size catalyst in the temperature range of interestfor FCC. The method employed proved to be adequate and sensitive for the quantification ofthese issues, which are important in reactor design and simulation and catalyst evaluationprocedures.